This invention relates to circuits for controlling the operation of a furnace and, more particularly, to circuits developed to prevent conditions which occur as a result of "lockout".
"Lockout" occurs where fuel, such as natural gas, fed to the burner fails to ignite or does ignite and then is subsequently extingished and the primary control system of the furnace shuts "off" the fuel supply thereto while the thermostat is still calling for heat. Since, in cooler weather, the temperature of the room will usually remain below the level set on the thermostat, the furnace will remain in this "lockout" condition until the primary control is reset, which is generally accomplished by interrupting power thereto by "opening" and then "closing" the thermostat.
In my U.S. Pat. No. 4,856,984, a control circuit is disclosed for automatically resetting the primary burner control after a "lockout" occurs but, in this system, no means is provided to monitor whether the primary control has turned "off" the fuel supply to the burner. The timer only monitors the thermostat operation and resets the furnace if the thermostat is "closed" and does not "open" again within a two-hour time interval. In addition, during such two-hour time period while the timer is running, the induced draft blower will continue to run and draw outside, cold air into the furnace and, as a consequence, accelerate cooling of the space being heated. Further, in some instances and for safety reasons, it is preferable to shutdown the furnace control when in the "lockout" condition and reset it manually.
Accordingly, the principal object of this invention is to provide a means for monitoring both the thermostat and the primary control system so as to shutdown the primary control after a short time interval when the furnace is in a "lockout" condition.
Another object of this invention is to provide means to shut "off" the induced draft blower motor when the fuel burner fails to ignite or the flame is extingished while the thermostat is still calling for heat.